Imbalance between autoreactive and regulatory T cell activities has been considered a major mechanism in the development of autoimmune disease. Previous studies on regulatory T cells have focused mainly on ab T cell receptor (TCR) expressing cells, commonly referred to as Treg cells. Recent studies have identified Gammadelta T cells as major regulatory cells in immune responses against tumor, infections, and autoimmune diseases; furthermore, manipulation of Gammadelta T cell activity has a beneficial effect on various immune responses, suggesting that immunologic interventions acting on Gammadelta T cells are promising avenues for immunotherapy. Nevertheless, the mechanism of Gammadelta T cell regulation is more complicated than previously thought and Gammadelta T cells can either inhibit or enhance an immune response; however, the mechanism leading to the opposite regulatory effects is poorly understood, which hampers the therapeutic use. Exploiting our established working model in EAU in which Gammadelta T cells either suppress or enhance the uveitogenic T cell response, we propose to determine the mechanism by which Gammadelta T cells up- and down-regulate autoimmune diseases. We will test the hypothesis that the regulatory function of Gammadelta T cells differs when they are activated to varying degrees or activated via different pathways by determining whether Gammadelta cells activated via different pathways exert different regulatory effect on generation of uveitogenic T cells and by determining how Gammadelta T cells alter their regulatory effect as a result of interacting with dendritic cells (DCs), natural killer T cells (NKTs), and ab T cell (Aim1). We also propose to determine the mechanism by which Gammadelta T cells regulate by examining the conditions that convert the Gammadelta T cells' up- and down-regulatory activity; to determine the reciprocal interaction between Gammadelta and DCs and between ab and Gammadelta T cells; and to determine whether abTCR+ Treg cells and Gammadelta T cells differ in regulating Th1 and Th17 responses (Aim 2). The proposed studies will test our hypothesis that the effects in Gammadelta T cells on immune responses are dynamic and can either be inhibitory or enhancing depending upon the activation status and size of the Gammadelta T cell population. Upon accomplishment of the study, the up- and down-regulatory effect of Gammadelta T cells should be better understood and we should be able to predict the relative role of differently activated Gammadelta T cells in the autoimmune responses with greater accuracy, which should greatly promote the establishment of a justified monitor system advising effective, Gammadelta-targeted immune-therapy.